This invention relates to an automatic thread handling system and, more particularly, to a system for controlling the needle thread for formation of each stitch.
In prior-art sewing machines, needle thread control was effected by tensioning the needle thread by a pair of discs mounted in the thread path between the thread supply and the eye of the needle. Such systems necessitate manual operation of a tension setting dial before sewing, so that a tension suitable for the impending sewing operation is applied by the discs to the needle thread. Thus, a complicated operation is required in preparation for sewing, and several trial sewing operations must usually be carried out to achieve neat stitches.
In order to overcome this problem, a variety of automatic thread handling systems have been devised, one example of which is disclosed in U.S. Pat. No. 4,215,641. The automatic needle thread control in these prior art systems is common in the following respects. That is, a thread-extracting mechanism is provided for extracting the needle thread from the thread supply and replenishing the thread used in the stitch formation. A computer calculates the length of thread to be extracted, based on work thickness and stitch length, and the resulting value is used to control the operation of the thread-extracting mechanism The thread thus extracted is taken up by a thread take-up mechanism including a take-up lever having a well-known characteristic illustrated by the solid-line curve in FIG. 4. The take-up mechanism is mechanically or electrically synchronized with the main shaft of the sewing machine for synchronizing the operation of the take-up lever with the vertical motion of the needle. The prior-art automatic thread handling systems are highly effective in reducing the work load of the operators in making preparations for sewing; however, the overall mechanical and electrical structures are rather complicated because of the necessity of incorporating the thread-extracting mechanism, in addition to the thread take-up mechanism, into the narrow space available within the head of the sewing machine.